1. Field of the Invention
The present invention relates to an intake system of a multiple-valve engine comprising a plurality of separate intake passages, which supplies intake air to a combustion chamber through a plurality of intake ports (which are "openings" in the present invention) each opened or closed by an intake valve.
2. Description of the Prior Art
Generally, in an engine having a cylinder in which a combustion chamber is formed at one end in a center line direction, it is known that a plurality of intake ports each opened or closed by an intake valve are opened to the combustion chamber of a cylinder and then intake air is supplied to the combustion chamber through the plurality of intake ports in order to increase intake efficiency of the combustion chamber and increase an engine output without increasing the capacity of the combustion chamber. Such an intake system of a multiple-valve engine comprises a common air intake passage formed corresponding to each cylinder and a plurality of air intake passages branching from a downstream end of the common air intake passage and connected to a plurality of intake ports of each combustion chamber.
According to the above multiple-valve engine, as disclosed in, for example Japanese Utility Model Laid Open Gazette No. 62-93122, when swirl of intake air is generate in order to improve combustibility in each combustion chamber, the downstream part of the common air intake passage corresponding to each cylinder is divided into three air intake passages and then intake air is supplied from each air intake passage into the combustion chamber through each intake port. In addition, it is proposed that control valves are provided on two of three air intake passages, whereby opening and closing operation of those passages are controlled to vary states of the intake air supplied into the combustion chamber through three intake ports.
However, since the plurality of air intake passages through which the intake air is supplied to the combustion chamber are controlled by the control valves in the prior art, the structure of the intake system becomes complicated and manufacture costs are increased. In addition, intake resistance is increased by the control valve and intake efficiency in the combustion chamber is reduced. Especially, when the engine is in a high load state, it is difficult to improve combustibility or anti-knocking function.